Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Wed May 28 2025
Day 1
Valid 16Z Wed May 28 2025 - 12Z Thu May 29 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
CENTRAL AND COASTAL TEXAS AS WELL AS CENTRAL AND SOUTHERN KANSAS...
...16Z Update...
...Kansas...
A new Slight Risk upgrade was introduced with this update for
portions of Kansas and far northwestern Oklahoma. This evening, a
southeastward moving vigorous shortwave trough will force
thunderstorms to form, likely in a line based on CAMs guidance, but
could evolve as multiple line segments. Meanwhile, an LLJ, though
not a particularly robust one, will stream Gulf moisture into the
region from the south. This LLJ will support backbuilding and
stronger storms capable of heavy rainfall. While individual storms
should be quite progressive as they track southeastward, any
backbuilding along the southern end of the line as well as
potential for new segments to form could increase the flash
flooding instances to more widely scattered frequency. Further,
NASA Sport soil moisture maps show that soils in the Slight Risk
area are at or above normal for moisture, which will convert more
rainfall to runoff.
...Texas...
The Slight Risk area inherited from the previous forecast remains
in place from the upper Texas Gulf Coast and Houston northeastward
into portions of central Texas. While the complex of storms further
south as moved into the Gulf, a remnant boundary on the eastern
edge of the return flow/LLJ off the Gulf will support renewed
widely scattered thunderstorm activity across this region this
afternoon into this evening. Soils here, like in Kansas, are also
at or above normal for moisture levels due to recent rainfall
activity. Thus, expect the storms, especially where they train, to
pose a widely scattered flash flooding risk.
Elsewhere, the Slight Risk was removed from the lower and much of
the middle Texas Gulf Coast as the complex of storms that caused
flash flooding in these areas earlier this morning has moved east
into the Gulf and no longer pose a flash flooding risk. Any further
convective development this afternoon should be isolated enough to
support only a Marginal Risk in this area.
...Mid-Atlantic...
The inherited Marginal Risk area was trimmed out of most of
Virginia with the exception of the Hampton Roads area. Instability
north of Hampton Roads is at or near zero, and therefore does not
support strong convective development. However, further south where
the Marginal remains for the Outer Banks and eastern North
Carolina, afternoon instability is expected to increase enough to
support convective development along a stalled stationary frontal
boundary draped over eastern North Carolina.
...New Mexico and West Texas...
The Marginal Risk area was removed with this update for this
region. CAMs guidance is showing only a mediocre signal in this
area, suggesting most areas see only convective showers at most. A
lack of moisture should preclude flash flooding. A stray instance
or two can't be ruled out, but was determined to be sub-Marginal.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Wed May 28 2025
Day 2
Valid 12Z Thu May 29 2025 - 12Z Fri May 30 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...
...Southeast U.S...
The combination of shortwaves ejecting eastward out of the Southern
Plains and the approach of a cold front from the north will lead to
ample ascent between sfc-500mb along a stretch of elevated moisture
between +1 to +2 deviations in PWAT anomalies. Consensus has
increased in a broad, scattered convective regime from the Lower
Mississippi Valley into the Southeastern U.S. leading a potpourri
of QPF maxima within areas of convective impact. Signals at the end
of the 00z HREF were quite bullish in the 12-hr window from 12-00z
Thu/Fri with neighborhood probabilities for >3" between 35-60%
across several small areas over the Central Gulf Coast, including
some "bullseye" patterns within a few urban centers between Baton
Rouge to Tallahassee. Thunderstorm concerns will continue until the
front passes through any specific area south of I-40 in the
period, so the prospects for local maxima capable of flash flooding
are elevated when you add the lower FFG indices currently in place
between MS/AL. This will come after seemingly 3 straight days of
heavy rainfall at any point in time for the region, so the FFG
recovery is pretty unlikely at this juncture. The previous SLGT
risk inherited was generally maintained, but did expand west given
the timing of the approaching shortwave out of TX and the
accompanying higher QPF signatures over LA between 12-18z Thursday.
...Ohio Valley...
A broad ULL positioned over the Midwest will pivot southeast with
a positioning across the Mid-Mississippi and Western Ohio Valley
through D2. Large scale forcing under presence of the ULL will
generate periods of convection across the above areas with a focus
on MO/IL/KY/IN. QPF maxima based on the ensemble bias corrected
output was between 1.5-2.5 over Central Ohio Valley with highest
signature over the Ohio River basin. The current outlook is still
within the MRGL risk threshold due to the lower instability
presence at the surface as much of the ascent is rooted in the mid
to upper levels given the ULL/trough influence. The previous MRGL
risk was relatively unchanged with only minor tweaks around the
periphery to account for the latest heavy QPF distribution.
...Texas...
More convection is expected over the Lone Star State on Thursday
afternoon through the end of the period. West TX will be the
initiation point with convection firing in-of the theta_E ridge
positioned over the Big Bend up through the Pecos River Valley. A
cold front will be pressing in from the north with a sharp cutoff
of any instability presence as MUCAPE will flat-line with fropa as
drier air advects behind the front. The best threat for convection
will likely occur over Southwest TX with some question as to
whether the cold front will subdue convective concerns for areas
north of I-20 by the second half of the forecast. Guidance is split
on how it wants to handle the shallow cold push with the NAM Nest
the most aggressive in its latitudinal push and globals lagging
enough to warrant storm threats as far north as the Caprock. For
now, left a broad MRGL over the region, but there will likely be
some fluctuations in the northern periphery of the MRGL risk area
in place. Will be monitoring over the next succession of updates.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Wed May 28 2025
Day 3
Valid 12Z Fri May 30 2025 - 12Z Sat May 31 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID ATLANTIC, OHIO VALLEY, AND SOUTHEAST...
A potent mid-latitude trough will pivot through the Ohio Valley on
Friday morning with a broad axis of diffluence downstream of the
mean trough leading to a large scale forcing regime that will
enhance regional precip field across the Mid Atlantic. At the
surface, low pressure will navigate into the Mid Atlantic with a
trailing cold front migrating eastward through much of the Eastern
CONUS, interacting with a wedge of low to mid-level instability
situated over the Central/Southern Appalachians and points east.
Greatest thermodynamic alignment will likely be closer to the
coastal plain, but elevated MUCAPE signatures can be found within
the Mid Atlantic Piedmont and under the primary trough. The
tandem of increased sfc-500mb forcing from the trough/front combo
will lead to a scattered to widespread convective pattern that will
induce locally heavy rainfall and potential for flash flooding.
Timing of the ascent pattern and frontal progression will be key in
determining the logistics of where the main axis of convection will
occur. As of now, the best chances for convectively driven heavy
rain are within the Central and Southern Appalachians, Southern VA
through the NC Piedmont, and the Eastern Carolinas up to the VA
Tidewater. These are the areas conducive for flash flooding at this
time with localized amounts reaching 2-3" in the places hit the
hardest. 1-2" is more likely out in the terrain of the Apps, but
that is a signal sufficient for flashier responses due to the
complexity of the topography. Despite the questions of timing, the
threat remains leading to a broad MRGL risk over much of the Mid
Atlantic with a southern inflection down towards FL as the trailing
front will move through the region and interact with a deeper
moisture layer and plentiful instability to induce some heavier
convective cores capable of flash flooding in more urbanized zones.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Wed May 28 2025
The flow pattern over our fine nation will be in transition into
the weekend. A series of leading shortwaves and secondary systems
will dig into an amplified upper trough over a cooled and unsettled
East as a trailing and rainfall focusing front works offshore into
the western Atlantic and the Gulf and Florida where some heavy
downpours may linger into next week with deepened moisture to
monitor. The bulk of upper trough energy lifts out early next
week. In this pattern a main deepened surface low will lift from
New England into eastern Canada this weekend. A WPC Day4/Saturday
Excessive Rainfall Outlook (ERO) Marginal Risk area remains in
place for northern New England in wrapped flow.
Meanwhile, upper ridging will prevail over much of a warmed West
into the weekend excluding upper trough energy with modest rain
working over the Northwest. However, guidance has trended in favor
of significant upper trough amplification into the West early next
week, along with the ejection of the lead southern stream closed
system from offshore Baja. This transition will act to provide
relief from the heat over the West with the building of much more
unsettled and increasingly widespread wet flow pattern spreading
increasingly inland and downstream to the Intermountain West and
especially the northern Rockies through midweek to include some
threat for elevation heavy snow.
In the West, upper ridging will support heat over the West this
into the weekend, with temperatures rising upwards to 10 to 20
degrees above normal. This in particular may lead to a hazardous
heat focus for the California Central Valley into the Desert
Southwest. Pre-frontal above normal temperatures closer to 10-15
degrees above normal should then spread to the north-central U.S..
Schichtel
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Wed May 28 2025
The flow pattern over our fine nation will be in transition into
the weekend. A series of leading shortwaves and secondary systems
will dig into an amplified upper trough over a cooled and unsettled
East as a trailing and rainfall focusing front works offshore into
the western Atlantic and the Gulf and Florida where some heavy
downpours may linger into next week with deepened moisture to
monitor. The bulk of upper trough energy lifts out early next
week. In this pattern a main deepened surface low will lift from
New England into eastern Canada this weekend. A WPC Day4/Saturday
Excessive Rainfall Outlook (ERO) Marginal Risk area remains in
place for northern New England in wrapped flow.
Meanwhile, upper ridging will prevail over much of a warmed West
into the weekend excluding upper trough energy with modest rain
working over the Northwest. However, guidance has trended in favor
of significant upper trough amplification into the West early next
week, along with the ejection of the lead southern stream closed
system from offshore Baja. This transition will act to provide
relief from the heat over the West with the building of much more
unsettled and increasingly widespread wet flow pattern spreading
increasingly inland and downstream to the Intermountain West and
especially the northern Rockies through midweek to include some
threat for elevation heavy snow.
In the West, upper ridging will support heat over the West this
into the weekend, with temperatures rising upwards to 10 to 20
degrees above normal. This in particular may lead to a hazardous
heat focus for the California Central Valley into the Desert
Southwest. Pre-frontal above normal temperatures closer to 10-15
degrees above normal should then spread to the north-central U.S..
Schichtel